Zero emissions sulphur recovery process with concurrent hydrogen production

ABSTRACT

Disclosed is a process for the concurrent production of hydrogen and sulphur from a H 2 S-containing gas stream, with zero emissions. The method comprises the thermal oxidative cracking of H 2 S so as to form H 2  and S 2 . Preferably, the oxidation is conducted using oxygen-enriched air, preferably pure oxygen. The ratio H 2 S/O 2  in the feedstock is higher than 2:1, preferably in the range of 3:1-5:1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/115,812 having an international filing date of 7 May 2012, nowallowed, which is the national phase of PCT applicationPCT/NL2012/050310 having an international filing date of 7 May 2012,which claims benefit of European application No. 11165177.4, filed 6 May2011. The contents of the above patent applications are incorporated byreference herein in their entirety.

FIELD OF THE INVENTION

The invention pertains to a process for recovering sulphur from aH₂S-containing gas stream, and to a sulphur recovery plant.Particularly, the invention pertains to the production of hydrogenassociated with a sulphur recovery process.

BACKGROUND OF THE INVENTION

Sulphur Recovery Plants are designed to remove H₂S from H₂S-containingacid gases from Amine Regeneration Systems and from Sour Water Strippersproducing sulphur, a non toxic product which can be stored and sold inliquid or in solid form to different users for several differentindustrial applications. The acid gases from Amine Regeneration Systemsand Sour Water Strippers, containing a variable amount of H₂S, aretreated in a Sulphur Recovery Unit (SRU), generally based on themodified Claus process, for bulk sulphur recovery and subsequently in aTail Gas Treatment (TGT) section for deep sulphur recovery. Otherimpurities contained in the sour gases, including ammonia andhydrocarbons, are destroyed in the Claus section.

The modified Claus process by itself recovers about 94÷96% (2 catalyticstages) or 95÷98% (3 stages) of the sulphur in the feedstock. A furthertreatment of the Claus tail gas is therefore necessary when a higherSulphur Recovery Efficiency (SRE) is required.

The modified Claus process comprises a sub-stoichiometric combustion ofthe acid gas stream in a thermal reactor (thermal stage) followed bycatalytic conversion in the Claus reactors (catalytic stage). In theClaus section one-third of the total H₂S is oxidized to SO₂, whichreacts with the remaining H₂S to form sulphur and water according to thefollowing reactions:

H₂S+1.5O₂→H₂O+SO₂ (oxidation reaction)  (1)

2H₂S+SO₂

1.5S₂+2H₂O (Claus reaction)  (2)

3H₂S+1.5O₂

3H₂O+1.5S₂ (overall reaction)  (3)

The goal of the process is to drive the overall reaction to nearcompletion. In the Claus thermal reactor, the H₂S contained in the acidgas is burnt with air (or with oxygen-enriched air in some specificcases) in a specific burner and only one-third of the total H₂S isoxidized to SO₂, while the remaining two-third is not reacted. The totalair amount is the one exactly sufficient to oxidize one-third of thetotal H₂S and to completely oxidize all hydrocarbons and ammoniacontained in the feedstock; the molar ratio H₂S/O₂ in the feedstock istherefore about 2:1 in order to get a ratio H₂S/SO₂ in the Claus tailgas of exactly, or as close as possible to, 2:1, which is thestoichiometric ratio for the Claus reaction, so maximizing SulphurRecovery Efficiency. During acid gas combustion, a small part of the H₂S(typically 5÷7%) is dissociated to hydrogen and sulphur as per followingreaction:

H₂S

H₂+0.5S₂ (dissociation or cracking reaction)  (4)

According to Clark et al., Alberta Sulphur Research Ltd. (ASRL),hydrogen formation also happens according to the following reaction:

4H₂S+O₂

2H₂+2H₂O+2S₂ (H₂ formation reaction)  (5)

Several side reactions are also involved, leading to the destruction ofammonia and hydrocarbons and to the formation of carbonyl sulphide COSand carbon disulphide CS₂. In order to complete the Claus reactions, asuitable residence time is necessary at high temperature in the thermalreactor.

The Claus thermal reactor is typically followed by a waste heat boilerwhere furnace effluent is cooled down to about 300° C. and heat isrecovered by raising high pressure steam and by a sulphur condenserwhere process gas is cooled down to sulphur dew point by raising lowpressure steam and liquid sulphur is separated.

The Claus thermal stage is generally followed by two or three catalyticstages, each one composed by a gas reheater to bring the gas to theoptimal reaction temperature, a catalytic reactor where the Clausreaction takes place and a sulphur condenser where gas is cooled andliquid sulphur is condensed and separated. The Claus reaction is anexothermic equilibrium reaction thermodynamically enhanced by lowtemperatures. The first Claus catalytic reactor is partly filled with aClaus catalyst (Alumina based) to enhance the Claus reaction and partlyfilled with a specific high conversion catalyst (Titania based) toenhance the hydrolysis of COS and CS₂. The second and third Clauscatalytic reactors, if any, are generally filled with Claus catalyst(Alumina based) to enhance Claus reaction.

In order to satisfy the >99% sulphur recovery efficiency normallyrequired for a Sulphur Recovery Plant, the Claus section is generallyfollowed by a Tail Gas Treatment section. Several different alternativeprocesses have been proposed over the years to boost Sulphur RecoveryEfficiency, like the SCOT method by Shell Oil Company, the RAR processby TKT, the CBA process by AMOCO, the CLINSULF/DEGSULF method by LindeActiengesellschaft or the BSR Selectox process by UOP. In thetraditional reductive Tail Gas Treatment section, the process gas from aClaus section is preheated and combined with hydrogen from an externalsource prior to being fed to a hydrogenation reactor, where all sulphurcompounds are converted to H₂S over a specific reduction catalyst (Coand Mo oxides based), which performs both the hydrogenation and thehydrolysis functions. The reactor effluent is cooled down in the quenchtower by means of circulating steam condensate. The H₂S produced in thehydrogenation reactor is recovered in an amine absorber with a specificamine aqueous solution and recycled to the Claus section from the top ofan amine regenerator, where the enriched solution is stripped.

The tail gas from the amine absorber is sent to a thermal incineratorfor the oxidation of residual H₂S and other sulphur compounds, such asCOS and CS₂, to SO₂ prior to disposal to the atmosphere via a dedicatedstack.

The main drawbacks of traditional Claus Plant are the need for large andexpensive equipment against very low sulphur economic value, continuousemissions of SO_(x) (SO₂ and SO₃), CO, CO₂, NO_(x) plus traces of H₂Sinto the atmosphere, and continuous import of hydrogen from the network,for process gas reduction in the TGT section.

In some Plants, where hydrogen is not available, for example in gasfields, the reducing gas mixture is generated in a reducing gasgenerator by sub-stoichiometric fuel gas combustion. The main drawbackof such alternative configuration is the 10÷15% higher process gas flowrate and subsequent larger equipment size due to substantial quantitiesof inerts coming from in-line fuel gas combustion (mainly nitrogen fromair and water and carbon dioxide from combustion).

Some alternative processes have been proposed over the years, which areaddressed to thermal or catalytic partial oxidation of H₂S.

U.S. Pat. No. 4,481,181 by GA Technologies Inc. discloses a process forremoving sulphur and recovering hydrogen from a H₂S-containing gasstream coupling thermal partial oxidation of H₂S to sulphur and waterand thermal dissociation of H₂S to hydrogen and sulphur in the samereaction zone, preceded by feedstock heating section and followed by acooling zone and by a sulphur condenser, using pure oxygen and asubstantial proportion of nitrogen with a H₂S/O₂ ratio in the feedstockbetween 10:1 and 25:1. The main goal of this patent is to thermallydecompose by partial oxidation and dissociation hydrogen sulphide intosulphur and hydrogen.

WO2010/036941 by Chevron U.S.A. Inc. and Drexel University discloses amethod for performing H₂S thermal dissociation at temperature below1600° C. based on H and SH radicals, in one embodiment over a suitableplasma catalyst.

Furthermore, Italian Patent 1 203 898 by Siirtec-Nigi discloses aprocess called HCR based on the operation of the traditional Clausthermal reactor at a slightly higher H₂S/O₂ ratio in the feedstock inorder to keep a H₂S/SO₂ ratio in the Claus tail gas significantly higherthan 2:1. The main goal of this process is to boost hydrogen productionin thermal reactor and to avoid hydrogen import in the TGT section. Alsowith such a process, Sulphur Recovery Plant emissions are not avoided.

From the above discussion, it is evident that several efforts have beenmade in the past, trying to propose a valid alternative to traditionalClaus Plant. In particular, some processes which have been proposed overthe years are based on the thermal or catalytic partial oxidation ofH₂S, while some other processes are focused on the thermal or catalyticcracking of H₂S. None of the proposed processes is conceived andarranged to perform H₂S conversion to hydrogen and sulphur aiming atfavoring both reactions at the same time.

It would be desired to reduce, and preferably avoid, emissions into theatmosphere. It would also be desired to reduce, and preferably avoid,the importation of hydrogen into the process. Particularly, it would bedesired to generate hydrogen, and to optimize the export of hydrogenfrom the process, yet with concurrent production of sulphur.

SUMMARY OF THE INVENTION

In order to better address one or more of the foregoing desires, theinvention presents, in one aspect, a method for the production ofhydrogen from a H₂S-containing gas stream, comprising subjecting the gasstream to thermal oxidative cracking so as to form H₂ and S₂.

In another aspect, the invention provides a plant suitable forconducting the thermal oxidative cracking of a H₂S-containing gasstream, said plant comprising an inlet for a H₂S-containing acid gasstream, an inlet for an oxygen-comprising stream, and a thermaloxidative cracking reaction zone comprising a mixing zone, preferablycomprising a burner, and a thermal reactor.

In another aspect, the invention relates to a method for the combinedproduction of hydrogen and sulphur from a H₂S-containing gas stream,comprising subjecting the gas stream to the aforementioned process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a simplified flow scheme of a typical traditional ClausPlant comprising a thermal stage, two catalytic stages, a subsequentreductive Tail Gas Treatment section, and a thermal incinerationsection.

FIG. 2 presents a simplified flow scheme of an H₂S Thermal OxidativeCracking Plant according to the invention, comprising a thermaloxidative cracking stage, optionally a Claus catalytic stage, and asubsequent reductive Tail Gas Treatment section.

DETAILED DESCRIPTION OF THE INVENTION

In a broad sense, the invention is based on the simultaneous occurrenceof cracking and partial oxidation of H₂S so as to provide concurrentproduction of sulphur and of a significant amount of hydrogen. Thisserves to address the problem of gas emissions into the atmosphere andproducing at the same time a valuable hydrogen export stream.

It is emphasized that the thermal oxidative cracking in accordance withthe invention is a fundamentally different process from both the thermalstage and the catalytic stage in an existing Claus-type process. Withreference to the reaction equations (1) to (5) mentioned above, theClaus processes are directed to driving the above reaction (3) to nearcompletion. The present invention is based on the judicious insight toprovide a process based on the side reactions (4) and (5), and topromote these reactions for the production, from a H₂S-containinggas-stream, of both hydrogen and sulphur.

In the present invention, a Thermal Oxidative Cracking (TOC) stagesubstitutes the Claus thermal stage. The process of the invention thusfavors H₂S dissociation and partial oxidation instead of completeoxidation and Claus reaction.

The thermal oxidative cracking is conducted in one or more reactionzones, preferably provided in a single reaction chamber. A singlereaction zone, in a single reaction chamber is preferred.

The invention presents the skilled person with the insight to promotethe above-mentioned reactions (4) and (5). The fact that thereto the gasstream is to be subjected to thermal oxidative cracking, implies a clearmessage to the skilled person as to how to carry this out.

The reaction is carried out at a temperature between 1100° C. and 1550°C., under the influence of oxygen. The ratio between H₂S and oxygen isabove 2:1, and preferably in the range of from 3:1 to 5:1, morepreferably in the range 4:1-4.5:1, and wherein the oxygen is provided ina gas stream comprising at least 40% of oxygen.

The Thermal Oxidative Cracking reaction zone is provided with oxygen.The oxygen is preferably provided as a gas enriched with oxygen ascompared to air. Preferably, this is an oxygen-containing gas-streamcomprising at least 40 vol. % oxygen, preferably at least 60 vol. %oxygen. More preferably, this oxygen is provided as substantially pureoxygen, viz. 90 vol. %-99 vol. % of oxygen, or as close to 100% asavailable.

The use of oxygen-enriched gas, and preferably pure oxygen, is not onlyrelated to optimizing the thermal oxidative cracking process, it alsopresents advantages such as the avoidance of unnecessarily largeequipment, which would be needed on account of the presence of largevolumes of inert (nitrogen) gas. Moreover, with reference to theinvention's purpose to produce hydrogen, in addition to sulphur recoveryand with reduced emissions, it will be advantageous to reduce, andpreferably avoid, the presence of nitrogen in the tail gas of theprocess.

The quantity of oxygen fed to the reactor is selected so as to achieve aratio H₂S/O₂ in the feedstock higher than typical figure of about 2:1.Preferably, H₂S/O₂ ratio in the feedstock should be in the range 3:1 to5:1, more preferably in the range 4:1-4.5:1, for example about 4.4.

In the preferred embodiment of operating the thermal oxidative crackingon the basis of a ratio H₂S/O₂ between 4:1 and 4.5:1, preferred reactiontemperatures to obtain simultaneously cracking and partial oxidation ofH₂S are in the range of from 1100 to 1400° C., preferably about 1200° C.

In one embodiment, the feedstock to the Thermal Oxidative Crackingreaction zone (H₂S-containing acid gas and oxygen-containing gas) ispreheated in order to increase the reaction temperature, to boosthydrogen production and to depress SO₂ formation. In a preferredembodiment, such preheating is performed with high pressure steam atabout 45 barg from a waste heat boiler in order to achieve a feedstockinlet temperature of about 240° C.

It should be noted that the reaction preferably is conductedautothermally. This refers to the fact that, whilst the process ispreferably adiabatic, heat exchange takes in fact place, since theoxidation reaction is exothermal, and the cracking reaction isendothermal, whereby heat made available through the exothermal reactionis utilized in the endothermal reaction.

All in all, the process of the invention is believed to favor reactions(4) and (5) relative to reactions (1) and (2), leading to lower H₂Sconversion, but on the other hand to significantly higher H₂ formationand to much lower SO₂ formation. As a consequence of the lower H₂Sconversion, a higher acid gas recycle rate from H₂S-containing gassource (e.g., an amine regenerator) to reaction chamber is obtained ascompared to a traditional Claus Plant.

The thermal oxidative cracking process of the invention is conducted atan optimum temperature so as to provide the minimum approach to maximumpossible equilibrium figures.

This results in increasing the hydrogen yield and minimizing SO₂formation, which in turn serves to minimize hydrogen consumption in theTail Gas Treatment section to reduce SO₂ to H₂S.

Preferably, the H₂S-containing acid gas and the oxygen-containing gasare mixed in a mixing zone prior to entering the thermal oxidativecracking zone. In one embodiment the mixing zone comprises a burnermounted in front of the reaction chamber.

The gas effluent from the reaction chamber is preferably quenched so asto avoid recombination of H₂ and S₂ to form H₂S, viz. by the inversereaction of (4), which would make the process sub-optimal in terms ofoverall conversion. Preferably this quenching is done substantiallyinstantaneously. The quenching is preferably to a temperature lower than950° C., more preferably in the range 850÷750° C. The residence time inthe quench zone is preferably as short as possible, typically of from 10ms to 300 ms, preferably from 10 ms to 100 ms, more preferably from 10ms to 50 ms.

The quench zone (which preferably is a zone of the reaction chamber) ispreferably followed by a waste heat boiler and a sulphur condenser tocool down the process gas and to recover liquid sulphur. The latter ispreferably done by raising high pressure steam in the waste heat boilerand low pressure steam in the sulphur condenser.

In a preferred embodiment, the quenching of the gas effluent from thereaction chamber is achieved by mixing with water in the final part ofthe reaction chamber. The mixing may be done by direct injection ofwater into the reaction chamber through a spray nozzle.

Although the process of the invention substantially reduces theformation of SO₂, it will be inevitable that some SO₂ is formed. Inorder to remove such SO₂, the Thermal Oxidative Cracking stage ispreferably followed by a Tail Gas Treatment section. Therein a part(e.g. about 10-15 vol. %) of the produced hydrogen is consumed in orderto reduce residual SO₂ to H₂S in a hydrogenation reactor. Due to themuch higher hydrogen content and to the much lower SO₂ content in thetail gas compared to traditional Claus Plant, the reduction step of theTail Gas Treatment section can be performed without any hydrogen import.

The tail gas is preferably preheated and fed to a hydrogenation reactor.Therein the SO₂, as well as other residual sulphur compounds, such asCOS and CS₂, are converted into H₂S, which is then removed. This removalcan be done in a conventional manner, e.g., by scrubbing the gas with alean amine solution in an absorber.

In one embodiment, the Thermal Oxidative Cracking stage is followed byone Claus catalytic stage, comprising a gas reheater, a Claus catalyticreactor and sulphur condenser, in order to convert most of the SO₂ intosulphur, thereby minimizing H₂ consumption for SO₂ reduction in the TailGas Treatment section.

In one embodiment, the hydrogen stream obtained from the TGT absorber issent to end users, like hydrotreaters, hydrocrackers orhydrodesulphurizers. It should be noted that the composition of hydrogenrich stream from the top of the TGT absorber may be different dependingon variables such as SRU feedstock quality, plant configuration andoperating conditions, and may include traces or percentages of H₂O, N₂,CO, CO₂, H₂S, COS and CS₂.

In a preferred embodiment, a hydrogen stream obtained from the TGTabsorber is further purified in a Hydrogen Purification section (forexample a Pressure Swing Absorber). It should be noted that, prior topurification, the composition of a hydrogen rich stream from the top ofthe TGT absorber may be different depending on variables such as SRUfeedstock quality, plant configuration and operating conditions, and mayinclude traces or percentages of H₂O, N₂, CO, CO₂, H₂S, COS and CS₂.

The purified hydrogen is sent to end users, like hydrotreaters,hydrocrackers or hydrodesulphurizers.

The invention, in one aspect, also relates to a plant suitable forconducting the thermal oxidative cracking of a H₂S-containing gasstream, said plant comprising an inlet for a H₂S-containing acid gasstream, an inlet for an oxygen-comprising stream, and a ThermalOxidative Cracking reaction zone. Preferably, the plant furthercomprises a gas quench zone.

In one embodiment, the Thermal Oxidative Cracking reaction chamber isrefractory lined in order to withstand temperatures up to 1550° C.

The invention will be illustrated with reference to the following,non-limiting Figures and Examples.

DETAILED DESCRIPTION OF THE FIGURES

Looking at FIG. 1, in a traditional Claus Plant, acid gas from one ormore Amine Regeneration Unit(s) 1 is fed together with acid gas fromSour Water Stripper Unit(s) 2 and with a combustion air stream 3 to athermal reactor burner (or Claus main burner) 4, directly connected to athermal reactor (or reaction furnace) 5, where one third of H₂S isconverted to SO₂ and all other compounds such as hydrocarbons andammonia are completely oxidized. The furnace effluent, after an adequateresidence time in the thermal reactor, is cooled down in a Claus wasteheat boiler 6, where heat is recovered generating high pressure steam.The process gas from the Claus waste heat boiler is fed to a firstsulphur condenser 7, where gas is cooled generating low pressure steamand sulphur 8 is condensed and is sent to degassing and storage. Theprocess gas from the first sulphur condenser is preheated in a firstClaus reheater 9 before entering a first Claus catalytic reactor 10,where the reaction between H₂S and SO₂ to produce sulphur vaporscontinues until equilibrium. The process gas from reactor 10 is sent toa second sulphur condenser 11, where gas is cooled generating lowpressure steam and sulphur 8 formed in the reactor is condensed and issent to degassing and storage. The process gas from the second sulphurcondenser is preheated in a second Claus reheater 12 before entering asecond Claus catalytic reactor 13, where the reaction between the H₂Sand SO₂ to sulphur vapours continues until equilibrium. The process gasfrom reactor 13 is fed to a third sulphur condenser 14, where gas iscooled generating low pressure steam (generally 4.5-6 barg), or lowpressure steam (generally about 1.2 barg) and sulphur 8 formed in thereactor is condensed and is sent to degassing and storage. Claus tailgas 15 from third sulphur condenser is sent to Tail Gas Treatmentsection.

Looking at FIG. 2, in a H₂S Thermal Oxidative Cracking Plant accordingto the invention, acid gas from one or more Amine Regeneration Unit(s) 1is fed together with acid gas from one or more Sour Water StripperUnit(s) 2 and with a pure oxygen stream 41 (or an oxygen-enriched airstream) to a Thermal Oxidative Cracking reaction chamber 42, where H₂Sis partially oxidized to S₂ and partially dissociated into H₂ and S₂,while all other compounds such as hydrocarbons and ammonia arecompletely oxidized and only a very small amount of SO₂ is formed. Thereactor effluent is cooled down in a waste heat boiler 6, where heat isrecovered generating high pressure steam. The process gas from the wasteheat boiler is fed to a sulphur condenser 7, where gas is cooledgenerating low pressure steam and sulphur 8 is condensed and is sent todegassing and storage; tail gas 15 from sulphur condenser is sent to aTail Gas Treatment section.

In one embodiment, the process gas from the waste heat boiler 6 is fedto a first sulphur condenser 7, where gas is cooled generating lowpressure steam and sulphur 8 is condensed and is sent to degassing andstorage. The process gas from the first sulphur condenser is preheatedin the first Claus reheater 9 before entering a first Claus catalyticreactor 10, where the reaction between H₂S and SO₂ to produce sulphurvapors continues until equilibrium, so removing almost all SO₂. Theprocess gas from reactor 10 is sent to a second sulphur condenser 11,where gas is cooled generating low pressure steam and sulphur 8 formedin the reactor is condensed and is sent to the degassing and storage.Tail gas 15 from the second sulphur condenser (or from the first sulphurcondenser in the first embodiment) is sent to a Tail Gas Treatmentsection.

In both Plant configurations shown in FIG. 1 and FIG. 2, and also in theembodiment of the present invention comprising a further Claus catalyticstep, tail gas 15 from final sulphur condenser is first preheated in thetail gas preheater 16. In the traditional Claus Plant, as shown in FIG.1, tail gas is mixed as necessary with hydrogen obtained from anexternal network 17, while in the novel H₂S Thermal Oxidative CrackingPlant according to the invention, as shown in FIG. 2, separate import ofhydrogen is not necessary, and tail gas is directly sent to ahydrogenation reactor 18. In the hydrogenation reactor (or reductionreactor) all sulphur compounds contained in the process gas areconverted to H₂S under slight hydrogen excess. The tail gas leaving thereactor is cooled down first in a TGT waste heat boiler 19 generatinglow pressure steam and then in a quench tower 20, where the process gascooling is achieved by circulation of the condensate 21 generated in thegas cooling. Quench water pumps 22 provide water circulation to thetower, while heat is removed from the system by a quench water cooler23. The excess sour water 24 generated in the gas cooling is sent tobattery limits for treatment in the Sour Water Stripper (SWS) Unit. Thecooled tail gas from the quench tower is fed to the absorber 25. Theabsorption of the H₂S contained in the tail gas is accomplished using aselective lean amine solution 26 coming from an amine regenerator 27.The rich amine solution 28 from the bottom of the absorber is pumped bymeans of the rich amine pumps 29 to a lean/rich amine heat exchanger 30,where the rich amine is preheated using as heating medium the hot leanamine from the bottom of the amine regenerator prior of being fed toamine regenerator 27 itself. The lean amine from the bottom of the amineregenerator is pumped by means of the lean amine pumps 31, is firstcooled in the lean/rich amine heat exchanger 30 and then in the leanamine cooler 32 prior of being fed to the absorber 25. The acid gas 33from the top of regenerator is recycled back to the Claus thermalreactor burner 4 in the traditional Claus Plant (FIG. 1), while it isrecycled to Thermal Oxidative Cracking reaction chamber 42 in the novelH₂S Thermal Oxidative Cracking Plant of the invention (FIG. 2).

In the traditional Claus Plant (FIG. 1), the tail gas from the absorber34 is sent to an incinerator burner 35, directly connected to anincinerator 36, where all residual sulphur compounds are oxidized toSO₂. The combustion of the tail gas is supported with fuel gascombustion, therefore a fuel gas stream 37 and a combustion air stream38 are also fed to the incinerator burner. The incinerator effluent (orflue gas) 40, after an adequate residence time in the thermalincinerator, is discharged into the atmosphere via a dedicated stack 39.In the novel H₂S Thermal Oxidative Cracking Plant (FIG. 2), the hydrogenrich stream from absorber 34 is sent to users outside the SulphurRecovery Unit.

In a preferred embodiment, the hydrogen rich stream from the absorber,containing some amount of impurities such as N₂, CO₂, H₂S, COS and CS₂,is sent to a further Hydrogen Treatment section 43, where it is furtherpurified. A substantially pure hydrogen stream 44 from HydrogenTreatment section is finally sent to different end-users.

Example 1

A possible plant configuration is the following. Acid gas and oxygen arefed to a reaction furnace where the oxidation reactions take place. Thereaction products are partially quenched and enter a Waste Heat Boilerfor the recovery of the reaction heat. After heat recovery, the processgas enters a Sulphur Condenser for the separation of the producedsulphur and for a further heat recovery.

The completion of the reactions is obtained in a catalytic block, whichconsists of a process gas preheater, a catalytic reactor and a finalsulphur condenser.

From the final condenser the tail gas is sent to a traditional reductiveTail Gas Treatment.

The selected conditions are the following:

H₂S/O₂ ratio 4.4

Adiabatic temperature 1200° C.

The feedstock has been preheated to 240° C.

Considering these operating conditions, the H₂S conversion is 56%, where

15.8% is converted to H₂ and S₂ according to reaction 4)

39.9% is converted to H₂O and S₂ according to reaction 3)

0.3% is converted to SO₂ and H₂O according to reaction 1)

while 44% of H₂S remains unconverted.

SO₂ is a substantial hydrogen consumer in the Hydrogenation Reactor andin order to reduce it to very low concentration, a Claus catalyticreactor has been considered downstream the Waste Heat Boiler and theSulphur Condenser.

The tail gas, coming out from the final condenser, after preheating isfed to the Hydrogenation Reactor, where the sulphur vapors aretransformed to H₂S, COS and CS₂ are hydrolyzed and CO is shifted tohydrogen.

The reactions are the following:

S_(n)+nH₂→nH₂S

COS+H₂O→CO₂+H₂S

CS₂+2H₂O→CO₂+2H₂S

CO+H₂O→CO₂+H₂

The remaining small concentration of SO₂ shall react as follows:

SO₂+3H₂→H₂S+2H₂O

The tail gas coming from the Hydrogenation Reactor is cooled down in aQuench Tower where the water generated in the oxidation reactions iscondensed.

Finally, the cool gas is washed in an Amine Absorber. From the top ofthe Amine Absorber a hydrogen rich stream, containing impurities such asH2S, CO₂ and N₂, is released.

The rich amine from the bottom of the Amine Absorber is sent to theAmine Regeneration section generating an H₂S and CO₂ stream, which isrecycled to the reaction furnace.

Therefore, the sulphur lost is only the H₂S contained in the hydrogenstream leaving the Amine Absorber, so the sulphur recovery efficiencycan be higher than 99.9%.

The balance shows that from a feedstock containing 100 kmol of H₂S, 30kmol of hydrogen can be recovered, leading to a good saving in hydrogenconsumption of the hydrotreating. It has to be noted that a traditionalplant has instead a hydrogen consumption of about 1-2 kmol per 100 kmolof H₂S for the Tail Gas hydrogenation step.

Other differences between a traditional plant and a plant with hydrogenrecovery can be noted by the comparison of the relevant heat andmaterial balances. For this purpose the material balance of the twoplant configurations has been carried out for a capacity of 100 T/D ofsulphur product. The relevant process gas flow rates in crucial parts ofthe plant are shown in Table 1.

TABLE 1 Process gas flow rates Traditional Plant H₂ Recovery Plant Kg/hKmol/h Kg/h Kmol/h Reaction Furnace 14425 481 9900 314 Final Condenser10255 422 5729 256 Quench Tower Outlet 8087 304 3938 157 Absorber Outlet7832 295 335 48

Table 1 shows that the process gas flow rates of the hydrogen recoveryplant are lower compared to the one of the traditional plant. Therefore,equipment sizes will be smaller and less expensive.

1. A plant suitable for conducting the thermal oxidative cracking of a H₂S-containing gas stream, said plant comprising an inlet for a H₂S-containing acid gas stream, an inlet for an oxygen-comprising stream, and a thermal oxidative cracking reaction zone comprising a mixing zone.
 2. The plant of claim 1 wherein the mixing zone comprises a burner.
 3. The plant of claim 1 wherein the thermal oxidative cracking reaction zone does not contain a catalyst.
 4. The plant of claim 1 wherein the thermal oxidative cracking is conducted according to the reactions H₂S

H₂+0.5S₂ (dissociation or cracking reaction)  (4) and 4H₂S+O₂

2H₂+2H₂O+2S₂ (H₂ formation reaction)  (5).
 5. The plant of claim 1 which further comprises a waste heat boiler for preheating.
 6. The plant of claim 1 wherein the thermal oxidative cracking reaction zone is autothermal.
 7. The plant of claim 1 which further contains a quench zone for effluent from the thermal oxidative cracking reaction zone.
 8. The plant of claim 1 which further contains a hydrogenation reactor for effluent from the thermal oxidative cracking reaction zone.
 9. The plant of claim 1 which further contains a subsequent Claus catalytic stage comprising a gas reheater, a Claus catalytic reactor and a sulfur condenser.
 10. The plant of claim 1 which further contains a hydrogen purification section.
 11. The plant of claim 1 wherein said thermal oxidative cracking reaction zone comprises a reaction chamber lined to withstand temperatures up to 1,550° C. 